Optical pulse radar equipment measures distances or ranges by applying the time delay principle. The time interval between a transmitted signal and a received signal, i.e., an echo or blip, is used as a measure of the distance from the transmitter-received to an object from which the signal was reflected. Between two electrical signals which are representative of the transmitted and received optical signals, pulses supplied by a fixed frequency crystal oscillator are counted in an evaluation circuit. The time of the transmitted signal is used as a start signal and the time of the received or echo signal is used as the stop signal for the evaluation circuit. Details of such a system can be found in the article "Measurement of Short Ranges with the Aid of Optical Pulsed Radar Equipment", Nachrichtentechnische Zeitscrift, No. 9, 535-540, 1973. The difficulty with this counting procedure is that there is no correlation between the transmitted signal which establishes the start line and which starts the pulse count and the edges of the pulses of the crystal oscillator, nor is there synchronization between the echo signal which ends the pulse count and the edges of the oscillator pulses. Thus, at the beginning and end of the pulse count there are residual times which are not taken into consideration and cannot be evaluated for range finding purposes. Each of these undetermined residual times is smaller than a full cycle of the oscillator pulse. Thus, ignoring these residual times is permissible for mesurements which are relatively coarse, i.e., in the kilometer and meter range, but for precise range finding, in which accuracies are to be kept in the centimeter and millimeter range, the previously mentioned measuring method is inadequate.
German OS No. 2,842,450 and U.S. Pat. No. 3,541,448 disclose methods and devices which make a coarse determination of the delay time by means of a coarse measurement and take account of the random residual times by means of additional precision instruments with the aid of precision measurement interpolators in order to derive the precise measured time from the rough measurement using a correction technique. However, the disadvantage of this equipment is the high apparatus expenditure. For mobile use, such as for use in portable range finders using this general principle, this prior art equipment is too heavy and large and is unsuitable under practical conditions. Furthermore, miniaturization cannot be achieved using currently available techniques.